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Kazuhito Ozawa, University of Tokyo, Graduate School of Science (Japan)
Tadamasa Ueda, Kyoto University (Japan)
Masaaki Obata, Kyoto University (Japan)
Giulio Di Toro, University of Padova (Italy)
Kyuichi Kanagawa, Chiba University (Japan)
Hiroko Nagahara, University of Tokyo (Japan)
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Ultramafic pseudotachylyte is thought to represent a remnant of earthquake rupture in the upper mantle, and potentially provides useful information for better understanding rupture mechanism of deep earthquakes (Karato, 2003). The presence of glass in the ultramafic pseudotachylyte provide definite evidence for frictional melting (Obata and Karato, 1995; Anderson and Austrheim, 2006), but it also implies very low ambient temperature not to induce extensive crystallization or devitrification, which casts doubt that the rupture event really took place in the upper mantle conditions. An exceptional case was reported by Ueda et al. (2006 JPGU Meeting), who found a thin ultramylonite in Balmuccia peridotite (Italy), exhibiting pseudotachylyte-like morphologies, such as injection veins branching out from a fault vein, but consisting completely of minerals without any glassy material or obvious quench textures. Obata et al. (2007 JPGU Meeting) further argued that the pseudotachylyte recrystallized at 700-800 °C in the spinel peridotite facies, and discussed the role of H2O-CO2 fluid played in the ductile-to-brittle transition leading a seismic rupture and frictional melting. However, these studies failed to provide conclusive evidence for melting or estimation of degree of melting, and the deformation history before and after the frictional melting has not been fully clarified yet. We have observed Cr-Al spinel in and bordering the holocrystalline pseudotachylyte with FE-SEM attached with EDS and EBSD, and obtained unequivocal evidence for melting and drastic transition of deformation phases through the formation and freezing of the pseudotachylyte.
Cr-Al spinel occurs in all micro-lithologies in and fringing the pseudotachylyte veins. Spinel shows enormous diversity in terms of morphology, Cr-Al zoning, and internal structure depending on its mode of occurrence. Clear evidence for melting comes from tiny spinel grains occurring in the injection veins. They are mostly elongate parallel to the adjacent foliation defined by pyroxenes and olivine and smaller than 20m across. It has a thin rim (< 2m in width) composed of fine intergrowth of clinopyroxene and spinel showing radial alignment. Spinel in the intergrowth is richer in Cr than the internal part. The internal part of spinel consists of several subgrains with similar crystallographic orientation with each other. The crystallographic orientation of spinel in the rim is nearly the same as that of the directly contacting subgrain. These textural and compositional features clearly indicate that Cr-Al spinel underwent partial dissolution by a short heating event accompanied by deformation in partially molten state. The present study demonstrates that the shear heating instability can be a mechanism of deep earthquake at mantle condition as high as 700-800 °C and ∼1.0 GPa following localization of ductile deformation induced by extremely high stress.
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